1. Field of the Invention
The present invention relates to a laser processing apparatus for performing laser processing along streets formed on a wafer such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, the front side of a substantially disk-shaped semiconductor wafer is formed with a plurality of crossing division lines called streets to thereby define a plurality of partitioned regions arranged like a matrix, and a plurality of devices such as ICs (Integrated Circuit) and LSIs (Large Scale Integration) are formed in these partitioned regions. The semiconductor wafer is cut along these streets to divide the partitioned regions, thus fabricating the individual devices. Further, an optical device wafer is fabricated by layering photodetectors such as photodiodes or light emitting devices such as laser diodes on the front side of a sapphire substrate. The optical device wafer is cut along streets to divide the individual optical devices such as photodiodes and laser diodes, which are widely used in electrical equipment.
As a method of cutting a wafer such as a semiconductor wafer and an optical device wafer along the streets, a method of forming a laser processed groove is known. In this method, a pulsed laser beam is applied to the wafer along each street to thereby form a laser processed groove along each street. The wafer is cut (broken) along each laser processed groove (see Japanese Patent Laid-open No. Hei 10-305420, for example).
In this method of forming the laser processed grooves by applying a pulsed laser beam along the streets formed on the wafer, the pulsed laser beam has an absorption wavelength (e.g., 355 nm) to the wafer, and the focal point of the pulsed laser beam is set on the subject surface of the wafer to be processed. However, the wafer has undulations, causing variations in thickness. As a result, the focal point of the laser beam applied to the wafer cannot be set on the subject surface of the wafer, so that a laser processed groove having a uniform depth cannot be formed along each street. Accordingly, in order to form a laser processed groove having a uniform depth along each street on the wafer, variations in thickness in a subject area on the wafer to which the laser beam is applied must be detected and laser beam applying means must be moved to follow such variations in thickness.
To solve this problem, there has been proposed a laser processing apparatus including height detecting means for detecting the height of the front side (upper surface) of a workpiece held on a chuck table by applying a laser beam of visible light to the front side (upper surface) of the workpiece to detect the height according to the light quantity corresponding to the area of reflection of the laser beam applied to the front side (upper surface) of the workpiece (see Japanese Patent Laid-open No. 2007-152355, for example).
According to the height detecting means disclosed in Japanese Patent Laid-open No. 2007-152355 mentioned above, the laser beam of visible light is not transmitted through the wafer as a workpiece in the case that the wafer is formed of silicon. Accordingly, in this case, the light quantity corresponding to the area of reflection of the laser beam applied to the front side (upper surface) of the workpiece can be accurately measured. However, in the case that the wafer is formed of a transparent material such as sapphire and quartz, the laser beam is reflected both on the front side (upper surface) of the workpiece and on the back side (lower surface) of the workpiece. Accordingly, in this case, the light quantity of the reflected light from only the front side (upper surface) of the workpiece cannot be measured. Thus, according to the height detecting means disclosed in the above publication, the height of the front side of the workpiece cannot be detected in the case that the workpiece is formed of a transparent material.
To solve this problem, the present applicant has proposed a height detecting apparatus capable of reliably detecting the height of a workpiece held on a chuck table even when the workpiece is formed of a transparent material in Japanese Patent Application No. 2007-231907. According to this height detecting apparatus, a detecting laser beam having a circular spot shape oscillated from detecting laser beam oscillating means is converted into a laser beam having an annular spot shape by annular spot forming means. This detecting laser beam having the annular spot shape is applied to the workpiece.
The detecting laser beam having the annular spot shape applied to the workpiece is reflected on the upper surface of the workpiece with an annular spot shape. In the case that the workpiece is transparent, the detecting laser beam having the annular spot shape applied to the workpiece is also reflected on the lower surface of the workpiece with another annular spot shape. In this case, the size of the annular spot shape of the reflected light reflected on the lower surface of the workpiece is larger than the size of the annular spot shape of the reflected light reflected on the upper surface of the workpiece. Accordingly, the reflected light having the larger annular spot shape reflected on the lower surface of the workpiece is blocked by a pinhole mask, and only the reflected light having the smaller annular spot shape reflected on the upper surface of the workpiece is passed through the pinhole of the pinhole mask. Then, the light quantity is detected according to the reflected light having the smaller annular spot shape reflected on the upper surface of the workpiece.
Further, an objective lens for focusing the detecting laser beam (having a wavelength of 635 nm, for example) is required to have a relatively large NA in order to clearly distinguish the size of the smaller annular spot shape of the reflected light reflected on the upper surface of the workpiece from the size of the larger annular spot shape of the reflected light reflected on the lower surface of the workpiece. On the other hand, an objective lens for focusing the processing laser beam (having a wavelength of 355 nm, for example) has a relatively small NA (e.g., 0.2). Accordingly, the detecting laser beam and the processing laser beam cannot be simultaneously applied along the same optical axis. That is, it is impossible to detect the height of the workpiece by using the detecting laser beam and simultaneously follow the focal position of the processing laser beam.